The present invention relates to an optical scanning apparatus for recording a latent image on a recording medium, and more particularly, to an optical scanning apparatus for modulating laser light beam using an optical modulator and scanning the recording medium with the modulated laser light beam.
An optical scanning apparatus has been used for various image recording apparatuses such as a digital photo printer and an electrophotographic printer. In the optical scanning apparatus, a laser beam modulated in accordance with an image to be recorded is deflected in a predetermined one-dimensional direction (main scanning direction) and a recording medium such as a photosensitive material is scanned by exposure to the laser beam to record a latent image or a visualized image on the recording medium.
A semiconductor laser is mainly used as a light source for the optical scanning apparatus. The intensity of a laser beam emitted from the semiconductor laser is modulated in accordance with recording data, thereby recording an image on the recording medium. The intensity modulation of the laser beam is performed by output amplitude modulation or pulse width modulation.
A photosensitive material to be exposed may be sensitive to a wavelength range other than the wavelength range of a semiconductor laser currently available. In the case of such a photosensitive material, it is necessary to use a solid laser instead of the semiconductor laser. However, it may be difficult to perform high speed modulation through direct control of the solid laser. Therefore, it is necessary to externally modulate the intensity of the laser beam using an acoustic-optical modulator (AOM) or the like. Even when the semiconductor laser is used, there is a case where it is required to modulate the laser beam at a higher speed than a modulation speed which can be controlled with the semiconductor laser in view of the improvement of print productivity. Even in such a case, it is necessary to externally modulate the intensity of the laser beam using the AOM or the like.
In order to diffract the laser light beam emitted from the laser light source in the AOM, the laser light beam is allowed to enter the AOM so as to condense the laser light beam in the vicinity of a crystal surface of the AOM. At this time, there occurs a phenomenon in which dust floating in the apparatus collects at a beam incident point of the AOM. The deposition of dust on the crystal surface of the AOM, in particular, a region through which the beam passes causes a reduction in transmission efficiency, that is, a reduction in the amount of light on the surface to be exposed. Therefore, it is likely to impair print quality. Such a dust collecting phenomenon is more remarkable in a laser having a shorter wavelength.
In a printer having a calibration function, the output of the laser light source is increased by the reduced amount of light on the surface to be exposed to increase the amount of emitted light, whereby the print quality can be maintained. However, when the amount of emitted light from the laser light source is increased, it is likely to shorten the service life of the laser light source.
The crystal surface of the AOM is very fine and is readily damaged. Therefore, even when trying to remove dust deposited on the crystal surface of the AOM, the operation for removing the dust deposited on the crystal surface requires a large number of man-hours and a high cost. In order to prevent the dust from depositing on the crystal surface, it is possible to provide a sealing structure for the inner portion of an apparatus into which the AOM is incorporated. However, the structure is complicated in view of, for example, wiring of a harness connecting the laser light source with a laser light source driving portion provided outside a casing accommodating the laser light source. Therefore, this is not desirable in that costs, the number of parts, and the number of assembling steps are increased.
When the AOM is used, laser light beam having a beam diameter of several tens of μm or less is allowed to enter the AOM to diffract the laser beam. Therefore, the laser light beam is made incident on the crystal surface of the AOM in a high energy density state, so the crystal surface and a coating film formed thereon are damaged in some cases. In addition, there occurs not only the deposition of dust floating in the apparatus but also the deterioration of the surface of the AOM which is caused by burning of organic gas or the like. Even in such cases, the reduction in transmission efficiency of the AOM also impairs the print quality.